The present invention relates to a ladder-type piezoelectric filter having a plurality of piezoelectric resonator elements for a series branch and a plurality of piezoelectric resonator elements for a parallel branch, wherein each of the series piezoelectric resonator elements has an electrostatic capacitance which is different from that of each of the parallel piezoelectric resonator elements.
There have been proposed various arrangements of such a ladder-type piezoelectric filter. An example of such conventional piezoelectric filter is disclosed in U.S. Pat. No. 4,398,162, in which a plurality of piezoelectric resonator elements each having a relatively smaller electrostatic capacitance are connected to a series branch, and a plurality of piezoelectric resonator elements each having a relatively larger electrostatic capacitance, are connected to a parallel branch. Similar filter arrangement is disclosed in U.S. Pat. No. 4,353,045.
Recently, a digital communication system has been provided for telephone sets for vehicles, MCA (multichannel access) transmitter or the like. There is an increasing demand that the ladder-type piezoelectric filter to be used for such a digital communication system should have an excellent group delay frequency characteristic.
It is known that the group delay frequency characteristic of the ladder-type piezoelectric filter can be improved by decreasing Qm value thereof. That is, FIGS. 1 to 5 show amplitude characteristics (A) and group delay frequency characteristics (B) of the prior art ladder-type piezoelectric filter of a nine-element type with respect to different Qm values of 100, 200, 300, 400 and 600, in which the piezoelectric resonator elements in the series branch have an electrostatic capacitance of 70-150 pF and the piezoelectric resonator elements in the parallel branch have an electrostatic capacitance of 400-600 pF. It is appreciated that the best group delay frequency characteristic may be obtained when the Qm value is set to 100 as shown in FIG. 1.
To the contrary, it will be seen that an insertion loss of the ladder-type piezoelectric filter may be increased as the Qm value is smaller. FIG. 6 shows the insertion loss (dB) for different Qm values determined using the same samples as mentioned above. As will be seen from FIG. 6, the insertion loss is abruptly increased in the vicinity of the Qm value of 200, and has an extremely high level in the vicinity of the Qm value of 100. In general, it is necessary that the upper limit of the insertion loss is practically less than 9 dB. In order to fulfill this demand, the Qm value should be set to 200 or more.
It is, therefore, difficult to provide a ladder-type piezoelectric filter which fulfills the requirements of an excellent group delay characteristic and a lower insertion loss.